It is a creature that has existed since the beginning of time along with The Angelus which is the bearer of Light. Both of them need Human vessels to show the true extent of their power so they are like Virus like Deity's that turn you Meta.

I like the darkness but the fact that they need a vessel is why i think that they aren't that great. If they could stand alone they would be way greater. But then again if you get the darkness you get pretty cool powers.